Exhaust gas from vehicles powered by gasoline engines is typically treated with one or more three-way conversion (TWC) automotive catalysts, which are effective to abate NOx, carbon monoxide (CO), and hydrocarbon (HC) pollutants in the exhaust of engines operated at or near stoichiometric air/fuel conditions. The precise proportion of air to fuel which results in stoichiometric conditions varies with the relative proportions of carbon and hydrogen in the fuel. An air-to-fuel (A/F) ratio of 14.65:1 (weight of air to weight of fuel) is the stoichiometric ratio corresponding to the combustion of a hydrocarbon fuel, such as gasoline, with an average formula CH1.88. The symbol λ is thus used to represent the result of dividing a particular A/F ratio by the stoichiometric A/F ratio for a given fuel, so that; λ=1 is a stoichiometric mixture, λ>1 is a fuel-lean mixture and λ<1 is a fuel-rich mixture.
Gasoline engines having electronic fuel injection systems provide a constantly varying air-fuel mixture that quickly and continually cycles between lean and rich exhaust. Recently, to improve fuel-economy, gasoline-fueled engine are being designed to operate under lean conditions. Lean conditions refers to maintaining the ratio of air to fuel in the combustion mixtures supplied to such engines above the stoichiometric ratio so that the resulting exhaust gases are “lean,” i.e., the exhaust gases are relatively high in oxygen content. Lean burn gasoline direct injection (GDI) engines offer fuel efficiency benefits that can contribute to a reduction in greenhouse gas emissions carrying out fuel combustion in excess air. A major by-product of lean combustion is NOx, the after-treatment of which remains a major challenge.
Emission of nitrogen oxides (NOx) must be reduced to meet emission regulation standards. TWC catalysts are not effective for reducing NOx emissions when the gasoline engine runs lean because of excessive oxygen in the exhaust. Two of the most promising technologies for reducing NOx under an oxygen-rich environment are urea selective catalytic reduction (SCR) and the lean NOx trap (LNT).
Urea SCR systems require a secondary fluid tank with an injection system, resulting in added system complexity. Other concerns for urea SCR include urea infrastructure, the potential freezing of urea solution, and the need for drivers to periodically fill the urea solution reservoir.
The exhaust of gasoline engines can be treated with a catalyst/NOx sorbent that contain alkali or alkali earth components (Ba, K, etc.), which stores NOx during periods of lean (oxygen-rich) operation, and releases the stored NOx during the rich (fuel-rich) periods of operation. During periods of rich (or stoichiometric) operation, the catalyst component of the catalyst/NOx sorbent promotes the reduction of NOx to nitrogen by reaction of NOx (including NOx released from the NOx sorbent) with HC, CO, and/or hydrogen present in the exhaust. However, the NOx absorbing components also react readily with sulfur oxides in the exhaust to form more stable metal sulfates, thus reducing the NOx storage capacity. Treatments in a reducing environment at high temperatures (>650° C.) are required to remove the sulfur from LNT catalysts and recover the NOx storage capacity.
FIG. 1 shows an exemplary engine exhaust system configuration often used in gasoline engines of the prior art. Specifically, FIG. 1 shows an engine exhaust system 100 comprising a TWC catalyst 120 downstream from a gasoline engine 110 via an exhaust conduit 115, an optional gasoline particulate filter 130 downstream from the TWC catalyst 120 via an exhaust conduit 125, and a SCR catalytic article 140 downstream from the TWC catalyst 120 and the optional gasoline particulate filter 130 via an exhaust conduit 135. The gasoline particulate filter 130 can be catalyzed with one or more platinum group metals, specifically palladium and rhodium.
To meet current governmental emissions regulations, there is a need for a technology that addresses NOx emissions and the sulfur poisoning of SCR catalysts in gasoline engine applications.